With increasing demands on communications in the world, and fast development of 3G and 4G networks now and in the future, the optical transceiver and fiber optics communication industries have started to focus on finding cost-effective solutions to improve the performance of communication systems and increase system bandwidths.
From the system application perspective, (i) full and effective use of fiber bandwidth and (ii) increases in the fiber data-carrying capacity are commercially feasible solutions. Wavelength division multiplexing (WDM) technology transports the carrier signals (e.g., carrying different information) having two or more different wavelengths together via a WDM multiplexer such as from a transmitter. Thus, over a single fiber, WDM technology ensures the transmission of a large capacity of information at the same time and provides high bandwidth for users through multiplexing different signals at different wavelengths, which carry different information. Optical carrier signals with different wavelengths can be separated by a WDM demultiplexer at the receiver for subsequent processing. An optical transmission network based on the WDM technology has the capacities of network reconfiguration and bandwidth expansion, and especially it has become a recent trend in high-speed video, voice and/or data transmission networks for the development and utilization of these capabilities.
Based on their different channel spacing, WDM can be classified into Coarse Wave Division Multiplexing (CWDM) and DWDM. Typically, CWDM has a channel spacing of 20 nm, whereas DWDM has a channel spacing of 0.2 nm to 1.2 nm. Since the channel spacing of CWDM is relatively wide in comparison to DWDM, light of 5 or 6 different wavelengths can be multiplexed on one fiber. In DWDM, approximately 80 wavelengths or data channels can be multiplexed into one optical data stream on one fiber. Therefore, the DWDM technology is widely applied in major networks of various service providers.
According to the International Telecommunication Union, Standardization Sector (ITU-T), standard wavelength spacing of the DWDM system must be a multiple of 0.4 nm (e.g., 50 GHz) or 0.8 nm (e.g., 100 GHz). For 100 GHz channel spacing, the transceiver requires a light source with stable operation wavelength and low chirp. As a result, a cooled Distributed Feedback (DFB) semiconductor laser or a Distributed Bragg Reflector (DBR) semiconductor laser, or a cooled DFB semiconductor laser monolithically integrated with an electro-absorption modulator (EM) can be utilized. These solutions provide desired wavelengths by adjusting the temperature of the laser to ensure an operation wavelength with no or minimal deviation, thereby avoiding optical crosstalk among the DWDM channels.
For 50 GHz channel spacing, maintaining the temperature of laser alone is not sufficient to avoid optical crosstalk because the wavelength coefficient versus the temperature of the semiconductor laser is 0.08-0.1 nm/° C. In addition, the performance characteristics of a thermistor, which is used as a standard temperature monitoring device for the feedback to temperature control loop, may gradually degrade with aging to cause changes in the temperature calibration value and then a change of the operation wavelength. Furthermore, a change in the temperature gradient inside the laser packaging can result in inaccurate output wavelengths because the required spacing among the 50 GHz channels may not be maintained. Typically, all of the above factors result in optical crosstalk among the DWDM channels.
In general, technology applied to DWDM semiconductor lasers for the 50 GHz channels must use a built-in wavelength locker. The wavelength locker is used as a data point for real-time calibration or locking to operation wavelengths. However, the wavelength locker increases both the size of the laser package and complexity of laser packaging, which then increases the cost of the DWDM laser. Therefore, a wavelength locker is generally not commercially feasible for a hot-pluggable transceiver.
DWDM transceivers, and more specifically, DWDM XFP transceivers using a cooled electro-absorption modulation laser (EML), achieve wavelength stability of approximately ±40 pm at the beginning of lifetime (BOL) and approximately ±100 pm at the end of lifetime (EOL). Typically, a 50 GHz channel spacing transceiver requires wavelength stability of approximately ±20 pm for compliance with BOL specification(s) and approximately ±50 pm for EOL specification(s). Currently, cooled EMLs can only be applied to DWDM XFP transceivers with 100 GHz wavelength channel spacing. State-of-the-art cooled EMLs generally cannot meet the requirements of 50 GHz channel spacing DWDM XFP transceivers.
FIG. 1 shows a block diagram of a DWDM XFP transceiver 100, in which a microprocessor (MCU) 120 comprising a processor core and a flash memory which stores XFP MSA registers 122, through a TEC control circuit 130, adjusts and controls a cooled DWDM EML transmitter optical sub assembly (TOSA) 110 that outputs optical data OUT according to one or more International Telecommunication Union (ITU) standards. However, conventional TEC control circuit 130 and APC control circuit 140 may cause the DWDM XFP transceiver 100 to fail to meet wavelength behavior requirements such as turn-on time in DWDM applications.
DWDM XFP transceiver 100 also includes an EML TOSA 110, which includes a distribution grating laser diode [DFB-LD] 115 and an electroabsorption [EA] modulator 112. The transmitter portion of transceiver 100 includes an EA modulation control block 150 that adjusts a bias voltage for the operating point of the EA 112, and an EML control circuit 160 that receives electrical data from the electrical interface 180 or a modified version thereof, through a Bias-tee circuit (not shown) and/or clock and data recovery circuit 165 to be applied to the EML TOSA 110. The receiver portion of transceiver 100 includes a receiver optical sub assembly (ROSA) 170 that includes a photodiode PD 172 and a transimpedance amplifier [TIA] 174, configured to receive optical data IN from the optical network and provide an electrical signal to the electrical interface 180 (which outputs electrical data EDOUT to an electrical device or network component).